Millions of people have been, and continue to be, infected by pathogenic Treponema, including the three Treponema pallidum subspecies and Treponema carateum. The resulting diseases-venereal syphilis, yaws, bejel, and pinta-are all chronic, potentially debilitating and disfiguring diseases. Globally, there are ~11 million new cases of syphilis annually: infectious syphilis has doubled in the United States since 2000, has re- emerged in Europe, and has increased 10-fold in China. At least 2.5 million cases of nonvenereal treponemal infections are estimated. While immunity to the homologous T. pallidum strain develops during infection, that immunity may be ineffective for other strains and is not cross-protective to other subspecies. Consequently, repeated infection is common, even after effective treatment, thus maintaining the infection within populations. Subtle antigenic differences, then, are key to protective immunity in the pathogenic Treponema. The pathogenic subspecies of T. pallidum are very closely related and comparative genetic studies have revealed that much of the genetic difference among the subspecies resides in the 12-member tpr gene family, whose encoded proteins are antigenic and several of which may be located in the outer membrane of the bacterium, poised for interaction with the host and the immune system. This application focuses on TprC and TprD which are predicted to be surface exposed, are highly immunogenic, and which contain amino acid regions that are distinct among subspecies and strains. Surface exposure of TprC and D is supported by computer algorithms, 3D protein predictions, and, most importantly, functionally by opsonophagocytosis assays. We hypothesize that antigenic differences, localized to surface exposed loops of TprC and D, have functional significance in immunity to the T. pallidum subspecies and relate to the lack of cross-immunity among subspecies and strains. We propose the following aims: 1) Identify potential surface-exposed regions of TprC and Tpr D in multiple subspecies and strains of T. pallidum; 2) Define infection-induced and immunization-induced T and B cell epitopes in TprC and TprD; 3) Determine the role of the distinct regions of TprC and D in functional immunity, using homologous and heterologous T. pallidum strains as the targets of the functional assays; 4) Determine whether there is a role for conserved regions of TprC and TprD in immunity. Identification of antigens that contribute to cross-immunity is a means of defining the protective antigens of the pathogenic treponemes. This knowledge is critical to understanding the continued transmission of treponemal infections within populations and to determining the components of an effective vaccine.